The water cycle refers to the continuous movement of water between the Earth's surface and atmosphere. The water cycle is driven by energy from the Sun, whose rays cause liquid water to heat and change into gas in a process called evaporation. As gaseous vapor rises and circulates in the atmosphere, it cools and changes back into liquid in a process known as condensation. Tiny droplets of water in the atmosphere accumulate to form clouds, which then return the water to Earth as precipitation, namely rain or snow.

If you had to guess how much of Earth's water is fresh water — the kind that humans and most other living things rely on to live — what would you guess? Remarkably, only 3 percent of Earth's total supply of 1,386 million cubic kilometers (332.5 million cubic miles) of water is fresh water. Of that 3 percent, nearly seven-tenths is trapped in the icecaps and glaciers found in mountainous regions and at the poles. Another three-tenths is stored in underground aquifers. About one-third of the fraction that remains — or 0.007 percent of all the water on the planet — comprises fresh water sources on the surface: the lakes, rivers, and swamps that we commonly see around us.

Although many humans value rivers and lakes as recreational opportunities, nearly every living thing relies on them for survival. For example, free-flowing rivers provide several necessities. They're an important source of drinking water and irrigation for communities. They carry sediments that help create coastal wetland features such as marshes that provide habitats for many animal and plants. They also renew the supply of oxygen that fish and other aquatic life need to breathe.

Pollution poses the largest threat to fresh water systems. Pesticides in agricultural runoff kill unintended organisms, and raw sewage from municipal sources spreads disease to people and animals that rely on the water. Protecting rivers from pollution, however, isn't the only challenge to be met. Even though Earth's supply of fresh water is continually being renewed through the water cycle, it is still in short supply in many places. Finding ways to conserve this precious resource is of utmost importance.

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